The invention relates to a method for catalyst-free production of amino-containing organosilicon compounds having a minimal proportion of Si—OH and Si—OR moieties by use of (aminoalkyl)alkoxysilanes.
Aminoalkylpolysiloxanes are widely used in industry. For instance, block copolymers are obtainable via polyaddition by use of isocyanates. A multiplicity of products having custom-tailored properties are thus obtainable by combining various polymeric blocks. Purity is very important for the aminoalkylpolysiloxanes used to construct block copolymers. Chain ends have to be highly amino-functionalized or high molecular weights cannot be achieved for the block copolymers.
Methods for preparing aminoalkylpolysiloxanes are already known.
Equilibration methods using 1,3-bis(3-aminopropyl)tetra-methyldisiloxane are described in U.S. Pat. No. 5,461,134 and in EP-B-0739372. These methods are inconvenient in that they require long reaction times. Equilibration methods also have the in-principle disadvantage that the products obtained have a comparatively high silanol fraction. Si—OH chain ends act, however, as chemically labile chain stoppers in a subsequent reaction with diisocyanates and thereby prevent the attainment of high molecular weights. A further disadvantage of unconverted Si—OH chain ends being present in the aminoalkylpolysiloxanes is that they can be the starting point for hydrolytic chain degradation. Materials of this type accordingly display lower stability in the presence of moisture.
US-A-2011/301374 describes a method whereby remaining Si—OH chain ends may be reacted with silazanes and thereby converted into the corresponding aminoalkyl groups. The achievement of high conversions, accordingly, requires two reaction steps; this greatly adds to the cost of production.
U.S. Pat. No. 4,633,002 describes a method for producing aminoalkylpolysiloxanes wherein silanol-terminated siloxanes are reacted with aminofunctional trialkoxysilanes in the presence of 0.4 to 1% of organometallic catalysts, in particular tin compounds.
This method is disadvantageous on account of the required high temperatures of about 275° C. which, as reported therein, lead to yellowing of the reaction product due to decomposition processes.
EP-A-0628589 describes a method for producing aminoalkylpolysiloxanes from silanol-terminated siloxanes, aminofunctional dialkoxysilanes and at least 0.01 to 1% of a combination between the basic catalysts barium hydroxide or strontium hydroxide on the one hand and sodium borate and/or sodium phosphate on the other. Especially the toxicity of the heavy metals barium and strontium is prejudicial to any industrial use of this method.
EP-A-1580215 describes a method for producing aminoalkylpolysiloxanes from silanol-terminated siloxanes and amino-functional dialkoxysilanes in the presence of 30 to 200 ppm of the basic catalysts sodium hydroxide, sodium methoxide or sodium siloxide.
U.S. Pat. No. 7,238,768 describes the production of aminofunctional polysiloxanes by reaction of hydroxyl-functional polysiloxanes with a deficiency of aminofunctional silanes, based on the Si—OH groups used, and in the presence of carboxylic acids. According to the prescription disclosed therein, the reaction of the (aminoalkyl)alkoxysilane with the Si—OH groups is accompanied by an in situ reaction of the Si—OH groups with the added alcohol to form Si—O-alkyl groups.
As stated therein, the aminoalkylpolysiloxanes obtained all without exception contain both Si—OH and Si—O-alkyl groups.
According to the prescription disclosed in U.S. Pat. No. 7,238,768, the reaction of the (aminoalkyl)alkoxysilane with the Si—OH groups (the actual chain-stopping reaction) is also accompanied by a chain-extending reaction of Si—OH units with Si—OH units, which eliminates water and which is likewise catalyzed by action of the acidic catalyst.
EP-A-1580215 further reports that the conditions disclosed in U.S. Pat. No. 7,238,768 result in the formation of products that have a yellow color and a short shelf life.
The method is accordingly unsuitable for producing amino-functional polysiloxanes having a minimal proportion of Si—OH groups and Si—O-alkyl groups. Not only the Si—OH groups but also the Si—O-alkyl groups act as a labile chain stopper since the latter, when stored in the presence of atmospheric humidity, gradually convert into Si—OH groups by hydrolysis and liberate volatile alcohols in the process, while the Si—OH groups thus generated have the abovementioned disadvantages.
The method is also unsuitable for producing aminofunctional polysiloxanes having a constant molecular weight, since chain condensation results in the formation of products of distinctly increased molecular weight. But a constant chain length is very important for the abovementioned uses in block copolymers, since it is responsible for the physical properties of the materials obtained.
Yellowing and reduced shelf life likewise reduce product quality.
U.S. Pat. No. 6,284,860 describes the reaction of OH-terminated organopolysiloxanes with di- and trialkoxy(aminoalkyl)silanes in the presence of Brønstedt or Lewis acids to form organopolysiloxanes containing about 30 to 60% of Si—O-alkyl groups.
According to the prescription given in U.S. Pat. No. 6,284,860, the reaction requires, based on the Si—OH groups present, at least stoichiometric amounts of the acid, which then remain in the reaction product as amine salts.
This method is accordingly unsuitable for producing amino-functional polysiloxanes having a minimal proportion of Si—O-alkyl groups. It is also unsuitable because the resultant high salt contents lead to inferior physical properties—such as rheology and visual transparency, for example—for the target products.
WO2013/160104 describes a method for producing amino-containing organosilicon compounds having a minimal proportion of Si—OH and Si—OR moieties by use of (aminoalkyl)monoalkoxysilanes in the presence of catalytic amounts of an acid. The method described therein is suitable for preparing aminoalkylpolysiloxanes which by virtue of their high purity have in-principle suitability for the uses mentioned at the outset.
Yet, particularly in the production of high molecular weight aminoalkylpolysiloxanes, even just very minimal amounts of the catalyst can have an adverse effect on the transparency of the material. A frequent haze due to the very finely divided catalyst is also still observable in the descendent products. Dehazing a high molecular weight and hence highly viscous material is technically far from straightforward. In many cases, haze appears with a time delay in the course of the storage of the material, meaning an unavailability of a consistently high quality of the material for the further reaction.